Ring laser gyroscopes are described, for example, in U.S. Pat. Nos. 3,373,650 and 3,467,472 which issued in the name of Joseph E. Kilpatrick; and in copending application Ser. No. 782,460, filed Mar. 20, 1977, which is assigned to the present Assignee.
The ring laser gyroscopes shown and described in the patents include a triangular block which forms a triangular-shaped ring laser cavity defined by three corner mirrors. A triangular-shaped block is preferred since it requires a minimal number of mirrors. The cavity is filled by a gas which comprises, for example, helium and neon. The laser usually operates at one of two wavelengths; specifically, either at 1.15 micrometers in the infrared spectral band, or at 0.63 micrometers in the visible wavelength region.
Through proper choice of the ratios of the two neon isotopes Ne.sup.20 and Ne.sup.22 in the gas mixture, two monochromatic laser beams are created. The two laser beams respectively travel in clockwise and counterclockwise directions around the triangular cavity in the same closed optical path.
With no angular motion about the input axis of the ring laser gyroscope, the lengths of the two laser beams are equal, and the two optical frequencies are the same. Angular movement of the prior art ring laser gyroscope in either direction about its input axis causes an apparent increase in the cavity length for the beam travelling in the direction of such angular movement and a corresponding decrease for the beam travelling in the opposite direction. Because the closed optical path is a resonant cavity providing sustained oscillation, the wavelength of each beam must also increase or decrease accordingly. Angular movement of the ring laser gyroscope in either direction about its input axis, therefore, causes a frequency differential to occur between the two beam frequencies, and which differential is proportional to the angular rate.
In accordance with the prior art practice, the two beams are extracted from the laser at its output mirror, and they are heterodyned in a beam combiner to produce an interference pattern. The interference pattern is detected by a photodetector which senses the beam frequency of the heterodyned optical frequencies of the two beams, and this beat frequency is a measure of the angular rate.
A difficulty arises in ring laser gyroscopes at low angular rates, in that the frequency differential between the two beams is small at the low rates, and the beams tend to resonate together, or "lock-in" so that the two beams oscillate at only one frequency. It therefore is impossible to read low angular rates because the frequency differential proportional to the angular rate does not exist.
It is the usual practice, as described in the patents, and in the copending applications, to introduce mechanical vibrations to the gyroscope to eliminate lock-in. However, the effects of these mechanical vibrations must be compensated in the output of the instrument, and such compensation introduces noise into the input which increases as a function of the back scatter.
Each mirror used in a ring laser typically has about one part per million backscatter, and a reflectivity of about 99.7%. It is backscatter which causes the ring laser gyroscope to lock in at the low angular rates, and it is most desirable to reduce backscatter to a minimum since, as mentioned above, the noise produced in the output due to the compensating equipment increases as a function of the backscatter.
A method for measuring backscatter in a linear laser is described in an article by I. L. Bershtein and D. P. Stepanov in Izevstiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Volume 16, No. 4, Pages 531-536, April 1973. This article describes a measuring system in which a linear laser directs a beam towards a test object, and in which a test object is vibrated at a particular frequency. The system produces an output which is synchronous with the vibration frequency of the test object. Any light back scattered into the linear laser beam produces an amplitude modulation on the output which can be detected to obtain a measurement of the back scatter.
A technique which can be used to measure the reflectivity of ring laser mirrors, and the like, is described, for example, in an article by Virgil Sanders in Applied Optics, January 1977, Volume 16, No. 1, Pages 19-20. The Sanders article describes a system in which the test object forms a part of a high-Q laser cavity. A Brewster's angle window, in which the angle of incidence can be varied, and a variable loss element, are used to measure the loss due to the test object. This is achieved by reconfiguring the cavity so as to exclude the test object. The resulting decreased losses in the laser cavity are then compensated by offsetting the Brewster's angle window to produce a loss corresponding to the loss when the test object was part of the cavity. The required adjustment of the Brewster's angle window is an indication of the reflectivity of the laser mirror.
A disadvantage in the prior art systems for measuring back scatter and laser mirror reflectivity is that the two measurements must be made individually by the different systems, and by different set-ups, which makes any correlation of the two measurements difficult and time-consuming.
Moreover, the technique described in the Sanders article for measuring mirror reflectivity is, itself, most difficult and time-consuming. This is because any slight misadjustment to the laser cavity causes the cavity gain to fall below unity which, in turn, causes the laser action to cease. When that occurs, re-alignment is especially difficult, since there are no indicators as to how to readjust the laser to restore the laser action.
An important objective of the present invention is to provide a relatively uncomplicated measuring system which is simple to operate and which is capable of producing backscatter and mirror reflectivity measurements in a single set-up and in a minimum amount of time.